FUDforum 2.6.12 has been released, for the most part it is the same as RC3 with a few minor fixes. Additionally this release addresses a minor security issue, details of which can be found below.

Changes:

Updated Russian translation.

Some minor code cleanup.

Fixed login redirection.

Fixed splitting of a topic into a new forum.

Security Disclosure

Credit for the discovery goes to Rasmus Lerdorf.

In pre-2.6.12RC1 versions of the forum the error_dialog() that is being used to log error messages stored the HTTP_HOST ($_SERVER['HTTP_HOST']) without encoding special characters and then displaying this information in the admin error log viewer control panel.
(The data is being stored inside a text file, so there is no danger of SQL injection).

Technically it shouldn't be an issue since the webserver supposed to ensure that the host only contains valid characters. Alas, like many assumptions this one was wrong. On Apache 1/2 the host is not being at all validated and can contain things like HTML data and still complete a request to the primary virtual host on that IP/Server.

This means that if you are using Apache and your forum is running on a dedicated IP address or is setup as a primary virtual host for an IP then it is possible to inject HTML into the admin error log viewer control panel by putting HTML into the HOST header of the HTTP request. However, even in Apache not all characters are allowed within the header and chars such as / and many others are disallowed. Which means the type of HTML that could be injected is fairly limited.

And it stops at that line. I don't really know in what state is my FUD, but I can connect on it, and it's written FUDforum 2.6.12 at the bottom.
I'm using PostGreSQL 7.4.6-3, with PHP 4.3.10-9 on a Debian Linux.

Hmm works fine here... that template is found inside post_proc.tmpl which is being inlined by isearch.tmpl.

Right, I look into './thm/default/tmpl/post_proc.tmpl', it is there, BUT, I'm not using this theme, but the path_info one (./thm/path_info/tmpl/post_proc.tmpl). I mean I use that thing that makes nice urls
This file is nearly empty.

Ok, I "sort of" finally managed to upgrade. The problem is that my default theme for my FUD is using path_info, and not 'default'. When looking into the templates for path_info, there's no trace of this 'post_html_quote_start_p1'.

So, I changed back (using the theme manager) to default (instead of path_info) and ran the uprade process, which worked fine. But I still can't use anymore the path_info stuff because as soon as something tries to rebuild that theme, it gives the same error.

To summarize, if you want to test by yourself this bug:
1) Install a version < 2.6.12 (I tried 2.6.7 and 2.6.9)
2) Change the default theme to use the template set 'path_info'. (rebuild it
3) Try now to upgrade using the 2.6.12 upgrade script

Note: I haven't changed any of the themes/templates in any way, they have always been the standard/default ones.